Advice needed from Minnesota

Hello. Me and my girlfriend are thinking of planning a road trip for the summer. I will be 21 and she will be 20 when we go on the trip, so we want to find some cool artsy attractions, without having to drive TOO far across the country. Neither of us have ever been on a road trip, so we're excited for the new experience, but we don't really know what we're doing or where we want to go. Does anyone have any advice for cool places to see for 2 young college-age kids to do in 3 or 4 days without getting in over our heads? Thanks for your help!